1. Field of the Invention
The present invention relates to a process for the production of dialkyl carbonates, particularly C1-C3 dialkyl carbonates wherein the reaction occurs simultaneously with separation of the reactants and the carbonate products. More particularly the invention relates to a process wherein alcohol is reacted with urea and/or alkyl carbamate in the presence of a complex compound catalyst. More particularly the invention relates to a process wherein feed stream impurities are removed to produce stable catalyst performance, improved reaction rates and trouble free downstream operation of equipment.
2. Related Art
Dialkyl carbonates are important commercial compounds, the most important of which is dimethyl carbonate (DMC). Dimethyl carbonate is used as a methylating and carbonylating agent and as a raw material for making polycarbonates. It can also be used as a solvent to replace halogenated solvents such as chlorobenzene. Although the current price of both dimethyl carbonate and diethyl carbonate is prohibitively expensive to use as fuel additive, both could be used as an oxygenate in reformulated gasoline and an octane component. Dimethyl carbonate has a much higher oxygen content (53%) than MTBE (methyl tertiary butyl ether) or TAME (tertiary amyl methyl ether), and hence not nearly as much is needed to have the same effect. It has a RON of 130 and is less volatile than either MTBE or TAME. It has a pleasant odor and, unlike ethers, is more readily biodegradable.
In older commercial processes dimethyl carbonate was produced from methanol and phosgene. Because of the extreme toxicity and cost of phosgene, there have been efforts to develop better, non-phosgene based processes.
In one new commercial process, dimethyl carbonate is produced from methanol, carbon monoxide, molecular oxygen and cuprous chloride via oxidative carbonylation in a two-step slurry process. Such a process is disclosed in EP 0 460 735 A2. The major shortcomings of the process are the low production rate, high cost for the separation of products and reactants, formation of by-products, high recycle requirements and the need for corrosion resistant reactors and process lines.
Another new process is disclosed in EP 0 742 198 A2 and EP 0 505 374 B1 wherein dimethyl carbonate is produced through formation of methyl nitrite instead of the cupric methoxychloride noted above. The by-products are nitrogen oxides, carbon dioxide, methylformate, etc. Dimethyl carbonate in the product stream from the reactor is separated by solvent extractive distillation using dimethyl oxalate as the solvent to break the azeotropic mixture. Although the chemistry looks simple and the production rate is improved, the process is very complicated because of the separation of a number of the materials, balancing materials in various flow sections of the process, complicated process control and dealing with methyl nitrite, a hazardous chemical.
In another commercial process dimethyl carbonate is produced from methanol and carbon dioxide in a two-step process. In the first step cyclic carbonates are produced by reacting epoxides with carbon dioxide as disclosed in U.S. Pat. Nos. 4,786,741; 4,851,555 and 4,400,559. In the second step dimethyl carbonate is produced along with glycol by exchange reaction of cyclic carbonates with methanol. See for example Y. Okada, et al “Dimethyl Carbonate Production for Fuel Additives”, ACS, Div. Fuel Chem., Preprint, 41(3), 868, 1996, and John F. Knifton, et al, “Ethylene Glycol-Dimethyl Carbonate Cogeneration”, Journal of Molecular Chemistry, vol. 67, pp 389-399, 1991. While the process has its advantages, the reaction rate of epoxides with carbon dioxide is slow and requires high pressure. In addition, the exchange reaction of the cyclic carbonate with methanol is limited by equilibrium and methanol and dimethyl carbonate form an azeotrope making separation difficult.
It has been known that dialkyl carbonates can be prepared by reacting primary aliphatic alcohols such as methanol with urea (1) in the presence of various heterogeneous and homogeneous catalysts such as dibutyltin dimethoxide, tetraphenyltin, etc. See for example P. Ball et al, “Synthesis of Carbonates and Polycarbonates by Reaction of Urea with Hydroxy Compounds”, C1 Mol. Chem., vol. 1, pp 95-108, 1984. Ammonia is a coproduct and it may be recycled to urea (2) as in the following reaction sequence.

Carbamates are produced at a lower temperature followed by production of dialkyl carbonates at higher temperature with ammonia being produced in both steps.

As noted the above two reactions are reversible under reaction conditions. The order of catalytic activity of organotin compounds is R4Sn<R3SnX<<R2SnX2, wherein X=Cl, RO, RCOO, RCOS. A maximum reaction rate and minimum formation of by-products are reported for dialkyl tin (IV) compounds. For most catalysts (Lewis acids), higher catalyst activity is claimed if the reaction is carried out in the presence of an appropriate cocatalyst (Lewis base). For example, the preferred cocatalyst for organic tin (IV) catalysts such as dibutyltin dimethoxide, dibutyltin oxide, etc. are triphenylphosphine and 4-dimethylaminopyridine. However, thermal decomposition of intermediate alkyl carbamates and urea to isocyanic acid (HNCO) or isocyanuric acid ((HNCO)3) and alcohol or ammonia (a coproduct of urea decomposition) is also facilitated by the organotin compounds such as dibutyltin dimethoxide or dibutyltin oxide employed in the synthesis of dialkyl carbamates. WO 95/17369 discloses a process for producing dialkyl carbonate such as dimethyl carbonate in two steps from alcohols and urea, utilizing the chemistry and catalysts published by P. Ball et al. In the first step, alcohol is reacted with urea to produce an alkyl carbamate. In the second step, dialkyl carbonate is produced by reacting further the alkyl carbamate with alcohol at temperatures higher than the first step. The reactions are carried out by employing an autoclave reactor. However, when methanol is reacted with methyl carbamate or urea, N-alkyl by-products such as N-methyl methyl carbamate (N-MMC) and N-alkyl urea are also produced according to the following reactions:

The dialkyl carbonate is present in the reactor in an amount between 1 and 3 weight % based on total carbamate and alcohol content of the reactor solution to minimize the formation of the by-products.
In U.S. Pat. No. 6,010,976, dimethyl carbonate (DMC) is synthesized from urea and methanol in high yield in a single step in the presence of high boiling ethers and a novel homogeneous tin complex catalyst.

The ether solvent also serves as complexing agent to form the homogenous complex catalyst from dibutyltin dimethoxide or oxide in situ.
The separation of materials involved in the DMC processes is very important for the commercial production of DMC for economic reasons. EP 0 742 198 AI and U.S. Pat. No. 5,214,185 disclose the separation of DMC from a vapor mixture of methanol and DMC by using dimethyl oxalate (DMOX) as extraction solvent. Because of the high melting point of DMOX (54° C.), using DMOX is inconvenient and adds an extra cost to the separation.
Both urea and alcohols are highly hygroscopic. Urea contains an ammonium carbamate impurity. Therefore, water and ammonium carbamate are impurities in urea and alcohol feed. It has been found that impurities such as water, ammonium carbamate, etc, in the urea and alcohol feeds cause catalyst deactivation and line plugging on cold spots in the cooling section (the condenser) for the overhead vapor stream from the reactor. Water causes the deactivation of catalyst containing alkyoxy groups, for example, the methoxy groups on the organotin complex compound molecules are highly reactive with water molecules resulting in hydrolysis of the bond between the tin atom and oxygen atom of methoxy group. Ammonium carbamate causes problems for controlling the backpressure in the dialkyl carbonate producing reactor and plugging the cooling system (condenser) of the product vapor stream from the dialkyl reactor, because of the deposition of ammonium carbamate.